1. Field of the Invention
The invention relates to a composition. In particular, it relates to a composition comprising an ethylene/propylene/diene terpolymer and a crystalline ethylene/olefin copolymer, which is suited for forming a tire sidewall, and will be described with particular reference thereto. It will be appreciated, however, that the composition has a variety of other uses, in particular, where ozone and crack resistance are sought.
2. Discussion of the Art
Rubber tires, such as pneumatic tires, include sidewalls and treads, among other components. Because of the different conditions to which these components are subjected, the composition of the sidewall is generally different from that of the tread. Sidewalls are continuously subjected to distortion under normal road operating conditions. The extensive flexing can cause the sidewalls to crack. In addition to flex cracking, sidewalls are also subjected to atmospheric chemical action such as ozone attack. The overall result is that the sidewalls are prone to erosion and degradation.
The addition of amorphous ethylene/propylene/diene (EPDM) rubber for various tire components, such as a sidewall component is known to reduce the need for antioxidants and antiozonants in the composition, primarily because of its superior resistance to ozone degradation. This results in an improved sidewall appearance. However, a disadvantage of EPDM elastomers is that they tend to have poor crack growth resistance and poor adhesion.
Various methods of improving crack growth resistance have been proposed, including the addition of precipitated silica in the presence of a silica coupling agent such as, for example, an organosilane polysulfide which contains at least about 3.5 sulfur atoms in its polysulfidic bridge (See U.S. Pat. No. 6,279,633). Addition of halobutyl rubber has been suggested to improve crack growth resistance, as disclosed, for example, in U.S. Pat. Nos. 4,224,196, 5,532,312, and 6,201,049).
The present invention provides a new and improved tire wall composition and method of preparation.